How can I copy the material assigned to grease pencil2.8 strokes to other strokes?
An answer can be found here: How assign different material to strokes in grease pencil mode
You can use the assign button in the material panel after selecting the stroke



To assign a material to several strokes (for example, set all blue stokes in black)


You can select the blue material in the materials panel, then select the black one in the drop-down menu





Or you can switch to edit mode, select the blue lines by clicking on "select" in the materials panel and then select the black material and click on "assign" (or you can go to Stroke menu -> assign Material)




But it is also possible to change the material of just one blue stroke. 

Just select it manually in the viewport and then assign the new materials according to the same procedure as above ("assign" in material panel or via the Stroke Menu)